Technical Article: Uncovering Feeding's Real Costs

When selecting a feeding system, you need to consider not just its initial cost but all the costs related to operating and servicing the system over its lifetime.

This article tells you how to evaluate a feeding system's total cost by counting up the systems acquisition, functional efficiency, and support costs in terms of your application. Included are easy-to-follow examples for computing feeding costs, a step-by-step strategy for minimizing ingredient costs, and a method for evaluating other intangible factors when making your final decision.

An Accurate Feeding System Pays for Itself by Reducing Ingredient Costs

A feeding system for dry bulk solids performs the important function of dosing ingredients into a food, plastics, pharmaceutical, or other product blend in exactly the right proportion. The system typically includes multiple feeders and a controller. Whether gravimetric or volumetric, an accurate, efficient feeding system can quickly pay for itself by reducing your ingredient costs and keeping your process line up and running. But many of us don’t consider these long-term cost savings when we shop for a feeding system. Instead, we look only at the feeding system’s purchase price.

One reason for this is that it's easy to get information about a feeding system's initial cost, but more difficult to evaluate the system's hidden costs and its potential savings in operating costs over time. By recognizing that the system's initial cost is only a small part of its total cost, you can take a "total cost approach" to evaluating system costs.

This approach analyzes the easy-to-count costs along with the costs that aren't so easy to count, and it provides a method for weighing the many intangible factors that go into any system selection. Best of all, the total cost approach is a decision tool that will give you confidence in your feeding system choice.

Let's start by identifying the three components that make up the feeding system's total cost:

Acquisition cost: This is the sum of the system's purchase price, installation cost, and initial operator training cost. It's a surprisingly small slice of your feeding system's total cost.

Functional efficiency: This component places a cost value on how well the feeding system controls the material feedrate as required by your application and how this affects ingredient and other costs. For example, a highly accurate, efficient feeding system can save significant ingredient quantities and minimize product waste, providing major cost savings.

Support cost: This component includes not only estimated maintenance and service costs for the feeding system but equipment cleaning and reconfiguration costs to accommodate recipe changes. These costs can be significant if you need to produce custom products because they often require shorter runs and frequent ingredient changeovers.

The following information explains how to evaluate each of these components when calculating a feeding system's total cost.

Acquisition cost The feeding system's acquisition cost is the easiest to compute. Once you’ve defined your application requirements and given them to the supplier of each feeding system you’re considering, each supplier will give you a price quotation. By adding the system's shipping costs to the supplier's quoted installation and training costs, you'll have the acquisition cost.

But you can't stop here. The acquisition cost alone is only a small part of the system's total cost. To understand why, consider this example: ABC Co. just bought a replacement feeding system with a $100,000 acquisition cost and an expected 10-year service life to process 1,000 lb/h of red, green, and blue versions of a product. The system will run three 8-hour shifts per day, 340 days a year. The ingredient changeovers required for switching between the red, green, and blue product runs reduce the process’s available uptime by 15 percent, for a total uptime of 85 percent.

The ingredients: average cost is $2 per pound, the rate of rejected product (that is, waste) is 1 percent, and the product will sell for $3.50 per pound.

By breaking the system's costs down per hour, you can easily see what proportion of the system's total hourly cost is acquisition cost:

The acquisition cost per hour is:
$100,000/10 years/340 days per year/24 hours per day = $1.23/h

As shown in the pie chart in Figure 1, acquisition cost accounts for only 0.06 percent of the ABC Co. feeding system’s total cost of $1,943.23 per hour. And you can see that ongoing hourly costs for ingredients, lost production, and waste quickly mount up over time, overshadowing the system’s one-time acquisition cost. You can calculate these ongoing hourly costs for your application as part of evaluating functional efficiency and support costs, which are the next two components in the feeding system’s total cost.

Cutting ingredient costsIt all starts with your application:
Does your application require a simple single-ingredient volumetric setup, a sophisticated multi-ingredient gravimetric blending system with greater accuracy, or something in between? And what is the application’s defined function — that is, what set of quality standards must the feeding system meet to ensure that your end product has consistent properties? For instance, if your product is a food, what taste, nutritive value, and shelf life does it require?

If the product is a plastic, what color, hardness, and surface properties must it have? If it’s a pharmaceutical, what efficacy, solubility, and stability does it require?
Some of these quality standards govern the feeding function. While standards for one application might specify only a target feedrate or basic proportion for each ingredient, the standards for a more critical application will place upper and lower statistical limits (that is, tolerances) on each ingredient feedrate to control the feedrate variability and ensure that the end product has the desired attributes. This means that you can minimize ingredient costs by adjusting the feedrate up or down for each ingredient to achieve the most cost-effective blend.

The role of feeder accuracyThe key factor in reducing ingredient costs is feeder accuracy. Feeder accuracy is measured by weighing each of a set of timed catch samples of ingredient taken from the feeder's discharge and determining the sample set's standard deviation from the sample mean (average).

Results from a feeder with a highly consistent discharge will show a small standard deviation with a narrow distribution of sample weights. This indicates the feeder is more accurate. Results from a feeder with a less consistent discharge will have a higher standard deviation and a broader distribution of sample weights, indicating this feeder is less accurate.

Feeder accuracy is typically cited in terms of repeatability (that is, how repeatable, or consistent, the feeder's discharge rate is, as determined in the test described above) at 2 standard deviations. Thus a feeder with a measured ± 0.3 percent standard deviation is said to have a repeatability of ±0.6 percent at 2 sigma.

So how can you use feeder accuracy information to minimize ingredient costs?